A digital subscriber line (DSL) is a technology for bringing high-bandwidth information to homes and small businesses over ordinary copper telephone lines traditionally used for voice communication through the Public Switched Telephone Network (PSTN). Digital data is transported across the PSTN by converting the data into an analog signal that is transmitted by varying, or modulating, the frequency, phase, amplitude or other characteristic of a carrier signal. The modulation is performed by a DSL modem attached to a standard telephone line referred to as a “local loop”. When analog signals are received from other DSL modems in the PSTN, the receiving DSL modem performs an opposite function by demodulating the received analog signal to convert it back into digital data. DSL modems are typically installed in pairs, with one of the DSL modems installed in a customer's premises and the other in the telephone company's central office servicing that customer's premises. The pair of DSL modems is connected to opposite ends of the same twisted pair telephone line comprising the local loop. xDSL refers to different variations of DSL and ADSL (asymmetric DSL), G.Lite DSL (ITU-T standard G-002.2), HADSL (high bit-rate DSL) and RADSL (rate adaptive DSL).
To protect the circuitry and prevent current leakage between the transmit ports and the receive ports, a hybrid circuit is used. The hybrid circuit allows the transmitted signal to be conveyed to the transmission line without leaking into the receive port and likewise allows the received signal to be conveyed from the transmission line to the receive port without leaking into the transmit port.
xDSL modems require electrical power to operate. In an effort to reduce the amount of electrical power consumed by xDSL modems, a technique called “active impedance” is sometimes employed. This technique has some disadvantages, including signal attenuation introduced in the receiver path. This signal attenuation is proportional to the level of active impedance generated. Because of the signal attenuation, the signal to noise ratio of the received signal degrades. While active impedance improves power consumption by a ratio of 1.8:1, it increases losses in the receiver path by 12˜15 dB. This results in a decrease in the maximum loop reach of the xDSL modem; i.e., the maximum loop is the distance between the modems in the central office and the customer's premises.
Several solutions have been tried in an effort to solve this drawback. One solution is to eliminate the active impedance generation, but this will increase the power consumption of the xDSL modem. Another solution is to increase the gain of the receiver path, but this does not affect the signal to noise ratio. Yet another solution is to improve the noise characteristic of the receiver, but this increases the complexity, cost and power consumption of the modem.
Accordingly, a need exists for a method and design that allows an xDSL modem to employ active impedance generation without affecting the signal to noise ratio and thereby degrading the effective reach of the signal.